In an electrophotographic image forming apparatus such as a laser printer, an electrostatic copying machine, a plain paper facsimile machine or a printer-copier-facsimile multifunction machine, an electrostatic latent image formed on a surface of a photoreceptor body by electrically charging the photoreceptor surface and exposing the photoreceptor surface to light is developed into a toner image with a toner, and a developing roller is used for the development.
There is a trend toward the use of a toner including more uniform, more spherical and smaller size toner particles or a polymeric toner. In order to impart such toner with higher electrical chargeability and efficiently develop the electrostatic latent image into the toner image without adhesion of the toner to the developing roller, it is effective to use, as the developing roller, a semiconductive roller having a roller resistance controlled at not greater than 108Ω in an ordinary temperature and ordinary humidity environment at a temperature of 23° C. at a relative humidity of 55%.
To meet various requirements imposed on the semiconductive roller, studies have been made on the type of a rubber component and the types and proportions of additives to be used for the production of the semiconductive roller, the structure of the semiconductive roller, and the like.
For example, the semiconductive roller typically has a nonporous single-layer structure, so that the semiconductive roller can be produced at a higher productivity at lower costs as having an improved durability and an improved compression set property.
It is considered preferable to produce the semiconductive roller by using a rubber composition containing a rubber component including at least an ion conductive rubber such as an epichlorohydrin rubber in order to suppress reduction in toner charge amount and toner transport amount to ensure higher quality image formation when the semiconductive roller is used as the developing roller.
Problematically, where the semiconductive roller is used as the developing roller, a formed image is liable to have a reduced image density. This is because the ion conductive rubber such as the epichlorohydrin rubber is highly adhesive to the toner.
Patent Document 1 proposes that titanium oxide functioning to prevent the adhesion of the toner is added to the rubber composition containing the ion conductive rubber as the rubber component for the semiconductive roller in order to suppress the image density reduction attributable to the toner adhesion to provide a proper image density.
If titanium oxide (a filler having a higher hardness) is added to the rubber composition in an amount sufficient to ensure the aforementioned function, however, the semiconductive roller is liable to have an increased hardness to cause additional problems. More specifically, the semiconductive roller is liable to deteriorate the toner to reduce imaging durability, or liable to have a reduced nip width when being in press contact with the surface of the photoreceptor body. Thus, the formed image is liable to have a reduced image quality.
The term “imaging durability” is defined as an index that indicates how long the image formation quality can be properly maintained when the same toner is repeatedly used for the image formation.
A very small part of toner contained in a developing section of the image forming apparatus is used in each image forming cycle, and the remaining major part of the toner is repeatedly circulated in the developing section. Since the developing roller is provided in the developing section and repeatedly brought into contact with the toner, whether or not the developing roller can reduce damage to the toner is a key factor to the improvement of the imaging durability. If the imaging durability is reduced, the formed image is liable to have white streaks in its black solid portion or have fogging in its marginal portion, thereby having a reduce image quality.
Patent Document 2 proposes a semiconductive roller having a double layer structure including an electrically conductive elastic layer, and a surface layer provided on an outer peripheral surface of the electrically conductive elastic layer, having a sea-island structure formed from a mixture of an acrylonitrile butadiene rubber (NBR) and a styrene butadiene rubber (SBR) incompatible with each other and imparted with ion conductivity by addition of an electrically conductive agent of an ion conductive type.
Examples of the electrically conductive agent of the ion conductive type include lithium perchlorate, sodium perchlorate, calcium perchlorate and perchlorates of long-chain-alkyl quaternary ammoniums.
It is conceivable to form the semiconductive roller having the single layer structure by employing the arrangement of the surface layer. In this case, it is possible to maintain, the roller resistance at a lower level by the addition of the electrically conductive agent of the ion conductive type while preventing the adhesion of the toner without the use of the ion conductive rubber.
In this case, however, the ion conductive agent should be added in a greater amount based on the overall amount of the rubber component in order to maintain the roller resistance at the lower level.
Therefore, when the semiconductive roller is continuously subjected to an electric field or a higher temperature, for example, an excess amount of the ion conductive agent is liable to bleed on an outer peripheral surface of the semiconductive roller. Problematically, the bleeding ion conductive agent is transferred to the photoreceptor body and the like to contaminate the photoreceptor body, thereby reducing the image quality of the formed image.
Patent Document 3 proposes a semiconductive roller having a double layer structure including an elastic layer formed from a mixture of an ethylene propylene diene rubber (EPDM), an NBR and an SBR and containing an electrically conductive carbon black as an electrically conductive agent of an electron conductive type, and a surface layer of a fluorine-containing material provided on an outer peripheral surface of the elastic layer.
Where the semiconductive roller is to be imparted with electron conductivity by using the electrically conductive carbon black alone as the electrically conductive agent, however, the semiconductive roller should have the layered structure including the surface layer covering the outer peripheral surface as described above to stabilize the roller resistance. That is, the semiconductive roller is not allowed to have a single layer structure, thereby requiring an increased number of production steps and an increased number of materials. Problematically, this correspondingly reduces the productivity of the semiconductive roller and increases the production costs.
Patent Document 4 proposes a semiconductive roller formed by using an SBR in combination with an epichlorohydrin rubber (ion conductive rubber) as a rubber component.
This arrangement provides the following effects:
The combinational use of the SBR reduces the amount of the epichlorohydrin rubber which may cause the adhesion of the toner, thereby suppressing the reduction in image density due to the toner adhesion.
It is possible to obviate the need for blending titanium oxide or to reduce the blending amount titanium oxide as compared with the prior art to impart the semiconductive roller with flexibility, thereby improving the toner imaging durability.